Add test for single-slot normalization style

I didn't know what this meant in detail, when I created this ticket. I just copied it from #3455496-5: Drop simple and single+simple slot normaliziation styles → .

Now I do., and this is covered by 📌 Create test for checking markup serialization Active .

At least: let's put it like this: The current setup of the CustomElement class/setSlot() method

• means/implies that in the vast majority of cases, setSlot() sets single-slot normalization style
• implies that this only influences JSON output, whgucg in some circumstances (i.e. code that does 'unexpected' things) can produce inconsistencies between JSON and Markup output.

If we determine that the last bullet point needs changing: 🐛 setSlot() + addSlot() + output inconsistency Active is open for that.